The present invention relates to a three-phase machine, which is used in electromechanical devices, among other things, as a drive unit or for positioning and machining work pieces. The running properties of such machines—such as quietness of operation and constant torque—are negatively influenced during operation by structurally dictated disturbance variables.
One of these disturbance variables is referred to as detent torque. The detent torque is based on a reluctance effect (reluctance=permeance) and is composed of a fundamental wave and harmonic waves. The periodicity of the fundamental depends on the geometry of the machine. The frequencies of the harmonics are integral multiples of the fundamental. In actual practice, the detent torque inhibits movement, particularly at the start of movement, when the secondary part (moving component) changes its position in relation to the primary part (stationary component). The most pronounced harmonic, also referred to as the dominant harmonic, is associated with a particular wavelength that determines the periodicity of the harmonic.
Another disturbance variable is the voltage that is induced in the stator windings. The induced voltage is triggered by the change in the flux linking and as a rule, due to the non-sinusoidal excitation field, indicates a more or less harmonic-laden signal shape. The measured harmonicity of the induced voltage is a measure for the torque ripple that is subsequently present during operation of the machine, even under load. The ripple has more or less influence on the precision of the executed movement as well as the service life of the machine and can be determined proportionally by means of the formula for calculating the K factor by setting the harmonic content in relation to the fundamental.
A wide variety of methods have already been developed for reducing the detent torque. For example in multi-pole, rotationally symmetrical, permanent magnet-excited machines, it is possible to reduce this disturbance variable by means of inclined magnetization or staggered rotor magnets and by means of inclined stator grooves. These methods, however, increase manufacturing complexity because they require additional work steps, which increases production costs.
Another scheme for reducing detent torque lies in varying the distance between the magnetic poles that generate the excitation field. This idea is described extensively in the publications Reducing Torque Ripple in PM Synchronous Motors by Pole Shifting, by N. Bianchi and S. Bolognani, in Proc. ICEM 2000, pp. 1221-1226, Helsinki: Helsinki University of Technology, 2000 and Design of Permanent Magnet Motors With Low Torque Ripples: A Review, by W. Cai, D. Fulton, K. Reichert, in Proc. ICEM 2000, pp. 1384-1385, Helsinki: Helsinki University of Technology, 2000. Assembled poles that are skillfully arranged in relation to one another can also reduce the detent torque. The patent application DE 41 33 723 A1 makes use of this method. The rotating field motor disclosed therein has an octagonal rotor with poles distributed over its circumference. Four poles comprise a unit. The distances of the poles from one another are uneven. There are a total of two units, each with 4 poles, whose detent torques overlap one another in a phase-shifted manner and thus reduce the resulting detent torque of the machine. In the approach demonstrated, only the fundamental of the detent torque is taken into consideration and it is not completely compensated for. Harmonics of the detent torque are not taken into consideration. It should also be noted that embodying the rotor in the way shown in the example results in an uneven air gap and the structure is not circumferentially symmetrical. An imbalance arises, which upon rotation, exerts a periodically overlapping interference force on the axle bearing. With low weights or small bores, this imbalance must be compensated for in order to prevent bearing damage.
The arrangement demonstrated in the application DE 41 33 723 A1 was also not tested with regard to the induced voltage generated in the winding of the primary part. Particularly when embodying a generator, however, it is desirable to have an output voltage that is sinusoidal as possible. The higher the harmonic content of the output voltage, the higher the divergence from the pure sinusoidal form. A motor, though, would require a torque that is as free of harmonics as possible. Approaches aimed at reducing harmonicity are discussed in the publication Magnet Shaping to Reduce Induced Voltage Harmonics in PM Machines With Surface Mounted Magnets, by Jaime De La Ree and Nady Boules in IEEE Transactions on Energy Conversion, vol. 6, no. 1, March 1991 or Induced Voltage Harmonics Reduction of PM Cylindrical Machines, in IEEE Transactions of Industrial Applications on Industry Applications, vol. 28, no. 3, May/June 1992. In these cases, the aim is to grapple with the problem in permanent magnet-excited motors by optimizing the dimensions and shape of the excitation magnets. It is known that the shape of the excitation magnets can be used to influence the signal shape of the no-load voltage, for example by using radially magnetized shell magnets. But particularly when constructing larger rotor diameters, it is no longer possible to use glued-on shell magnets to reduce the induced voltage because they contribute to high eddy current losses. Larger rotors can also be embodied using inexpensive segmented flat magnets to limit eddy current losses. But the change in the magnet shape then results in a further increase in the harmonic content of the induced voltage.
The magnet width can also be used to influence and reduce harmonics. This approach is taken by the application EP 1130747 A2 in that the magnitude of an angle defined between the rotor axis and the two sides of a magnet embedded in the stator is varied in order to influence the field distribution. Among other things, this optimizes the induced voltage and the detent torque. In actual practice, though, this leads to complex geometrically required manufacturing procedures. The aim, however, is to use standard magnets that can be purchased in bulk and used for a wide variety of machine geometries, independent of the dimensions of the machine.